Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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Table IV. Operating Parameters and Applications for Acid Pickling Steel and Stainless Steel by Immersion Acid % v/v Hydrochloric 20-50 Sulfuric Phosphoric 20-40 25-45 Temperature, O 80-120 90-150 90-150 F Fluorides, oz/gal Wetters, oz/gal 3-5 1.5-3.0 0.5-2.0 0.01-0.03 0.01-0.03 0.01-0.03 Inhibitors, oz/gal 0.002-0.004 0.002-0.004 0.002-0.004 lic smuts and scales do not respond well to the acid dip. A more aggressive treatment is supplied by the acid pickle. Some heat-treat scales, weld scales, rusts, and oxides (formed by buffing or polishing operations) are readily attacked and removed in an appropriate acid pickle bath. The solution will also perform the functions of the acid dip. A typical acid pickle contains inorganic acids, such as hydrochloric, sulfuric, hydrofluoric, and phosphoric; and deflocculents, wetting agents, and inhibitors. In some immersion or electrolytic applications scales are seen to literally peel off in sheets and rusted surfaces "whiten." The acid pickle, when used in double- cleaning cycles (immersion or electrolytic), is usually the first acid step, fol- lowed by the second electrocleaner and a final acid dip. DESCALING ACID This process bath is a step above the acid pickle in regard to chemical strength and application. The general focus is removal of heavy deposits: rusts, scales (hot forging or rolling, casting), and other heat treatments. The bath constituents are similar to the acid pickle, but contain higher levels of accelerators such as fluo- rides and chlorides. Operating parameters: concentration, temperature, and time, may be increased when compared to the acid pickle. Descaling acids can be used by immersion or electrolytically. These baths are sometimes used to pretreat parts off line before processing in a standard finishing line. The pickling requirement is a critical factor of time, temperature, and con- centrations of the components. Times may range from 5 to 30 minutes. This makes the incorporation of an inhibitor very important. Surface pickling can be accomplished with minimal attack on the base metal, while also greatly mini- Table V. Operating Parameters and Applications for Acid Pickling Steel and Stainless Steel Cathodically Sulfuric Acid, % v/v 5-10 4-6 V. 30-50 A/ft2 Temperature, O F 90-120 Fluorides, oz/gal 1.5-3.0 Chlorides, oz/gal 3.0-4.0 . Use chemically pure lead anodes or carbon. For any cathodic acid application using carbon anodes in the presence of fluorides, the best resistant grade of carbon to fluoride attack should be used. Table VI. Operating Parameters and Applications for Descaling Steel and Stainless Steel by Immersion Acid 1. Sulfuric or 2. Hydrochloric 114 Acid Concentration, % v/v 7-12 7-12 Temperature, O F 90-160 90-160 Fluorides, oz/gal 4.0-6.0 4.0-6.0 Chlorides, Wetters, oz/gal oz/gal 6.0-8.0 — Inhibitors, oz/gal 0.01-0.03 0.002-0.004 0.01-0.03 0.002-0.004 Wetters, oz/gal 0.01-0.03 Inhibitors, oz/gal 0.002-0.004

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